Chinese family members of missing Malaysia Airlines MH370 passenger, are escorted away from the media outside the media conference area at a hotel near Kuala Lumpur International Airport.

Investigators probing the disappearance of a Malaysia Airlines jetliner with 239 people on board believe it most likely flew into the southern Indian Ocean, a source close to the investigation said today.

No wreckage has been found from Flight MH370, which vanished from air traffic control screens off Malaysia's east coast at 1:21 a.m. local time on March 8, less than an hour after taking off from Kuala Lumpur bound for Beijing.

An unprecedented search for the Boeing 777-200ER is under way involving 26 nations in two vast search "corridors": one arcing north overland from Laos towards the Caspian Sea, the other curving south across the Indian Ocean from west of Indonesia's Sumatra island to west of Australia.

"The working assumption is that it went south, and furthermore that it went to the southern end of that corridor," said the source, who spoke on condition of anonymity.

The view is based on the lack of any evidence from countries along the northern corridor that the plane entered their airspace, and the failure to find any trace of wreckage in searches in the upper part of the southern corridor.

Some sources involved in the investigation have voiced fears it could be drifting towards deadlock due to the reluctance of countries in the region to share militarily sensitive radar data or allow full access to their territory.

"These are basically spy planes, that's what they were designed for," said one source close to the investigation, explaining the hesitance of some nations to allow maritime surveillance aircraft into their waters.

Malaysia's Acting Transport Minister Hishammuddin Hussein, who is in charge of the operation, told a news conference that: "The search for MH370 involves diplomatic, technical and logistical challenges."